Secondary charged particles are emitted from a surface of the specimen to be examined when electron or ion source impinges on the surface with sufficient energy. Since the energy and/or the energy distribution of such secondary charged particles offers information as to the topography of the specimen, detectors are employed to detect the secondary charged particles and convert them to electrical signals used to generate images of the specimen. Scanning electron microscopes (SEMs) are generally provided with such a detection device for secondary electrons.
Specifically in SEMs, an electron beam source generates a primary electron beam passing through a hole in the middle of a detection device. A variable electrostatic or magnetic field deflects the primary electron beam to scan it over a region of a specimen. When the primary beam strikes the specimen, secondary electrons are generated. Such particles have an energy which is significantly lower than that of the particles in the primary electron beam, e.g., 50 eV. Some of these secondary electrons pass back up through the electron optical column and interact with the primary beam deflection fields as they pass back through the optical column. Thereafter these secondary electrons are imaged onto the detection device.
To collect as many of the secondary electrons as possible, conventional SEM systems use secondary electron detectors that are relatively large. However, since leakage current and capacitance of a detector (e.g., a photodiode) are proportional to the detector area (e.g., the area of the photodiode depletion zone), it is not desirable to have an excessive detector area. In addition, conventional secondary electron detectors are typically rotationally symmetric. However, while the detector may be rotationally symmetric, the pattern of electrons landing on the detector might not be symmetric. Secondary electrons traveling back up through the optical column tend to undergo rotation and deflection as a result of magnetic fields used in immersion lenses or the deflection by the optical column. The amount of rotation depends partly on the landing energy of the primary electrons. Higher energy electrons undergo less rotation since they spend less time in the magnetic field due to their relatively high velocities.
Conventional detectors generally include a metal shield to cover up any exposed dielectric on the detectors to prevent charging that would produce a deflection field. For example, prior art SEM systems use a MEMS-type shield made of thin metal foil that is patterned (e.g., by laser) and bonded to the detector. The shield and the detectors are conventionally manufactured separately and are integrated afterwards. Thus, conventional detectors require additional process steps that confer additional cost and complexity of alignment or assembly.
It is within this context that aspects of the present disclosure arise.